It is common for modern telephones to have a display capable of displaying text. The text may be received by the telephone over a telephone line from an application or script running on a remote source, or may be generated by an application running on the telephone itself which was downloaded from a remote source. Typically, the telephone is only capable of receiving and displaying standard ASCII characters which are represented by a character code consisting of a byte with a value from 00 hex to FF hex.
The default character set for one existing display based telephone design consists of the extended ASCII character set which includes all standard characters, plus international characters (to support languages that require accents), as well as box drawing characters for surrounding text with a single or double line. A disadvantage with this standard character set is that it does not allow any application defined graphics to be displayed.
The current ADSI (analog display services interface) protocol provides a standardized method of downloading characters in the standard character set to a display based telephone set, but does not provide for any way to display graphics on the display based telephone set.
In Canadian patent application 2,188,269 to Keen, published Jun. 8, 1995 a method is disclosed for displaying more than one font on a limited dot matrix display device. In addition to displaying characters in a standard font using all the pixels in a five by seven pixel matrix, a reduced four by six matrix is used to display characters with the pixels of the five by seven matrix left over being made available for underlining, inverse video etc. Adjacent five by seven pixel matrices are separated by a space on the display such that adjacent characters do not touch. This method does not provide for remote application specific redefinition of characters, and does not provide a way to display contiguous graphic images.